Presumably you are on a DONEx or AONEx, in which case L is the correct booking class for most flights when A/D is not available. Y is only for some specific routes, such as AE one-class (regional) flights in the US and some other one-class flights. It is spelled out in the OWE rules as to which book into Y (very few); but if you downgrade from A to coach because of availability on other flights, it must be booked in L and not Y! This is one of the booking mistakes that will be caught in a re-issue if you switch carriers (so maybe staying with AA isn't so bad after all?). Perhaps not very nice of the rules to not allow Y when downgrading, but these are the rules you bought into. Often airlines will let you fly standby and book you into J/Y inventory when standing by at the gate, however that is a favour and not a rule you can depend on.